a) Technical Field
The invention is directed to a process, a device and an installation for the production of laminates, for example, laminates which can be hardened or cured by radiation, in which adhesive is distributed on a thin film and the film is combined with an individual sheet or a continuous sheet by means of a laminating machine to form a laminate. Such laminates are used particularly in advertising media or in the packaging industry.
b) Background Art
The relatively thin film of plastic material is taken off from a roll in a continuous manner. When individual sheets are used, the sheets are taken from a stack. The film and sheet are fed together to the laminating machine as a laminate. In order to prevent soiling of the rollers in the laminating machine, the individual consecutive sheets are often overlapped in the edge region. The overlapped or underlaid sheets are coated with the film. However, it is also possible to laminate the sheets end to end. The individual sheets are joined with one another by the film after laminating. Before processing further in a subsequent working process, the laminate must be separated again into individual laminated sheets.
DE-PS 2 817 917 shows a known solution for the use of radiation-curable adhesives, so-called UV adhesives. This solution represented significant progress over conventional solutions in which the adhesive had to be dried over a relatively long period of time, partially by infrared rays and hot air. The laminate may sometimes take up to 24 hours to dry before further processing is possible. In the case of conventional adhesives, the processing speed of the known installations must be under 40 meters per minute, for example, since operating troubles would otherwise increase substantially. 60 meters per minute was formerly assumed as a "processing sound barrier", as it were.
When using adhesives which can be polymerized or cured by UV rays or electron rays, the adhesive need not be dried in the strict sense, but rather hardened or cured, which takes only fractions of a second. Immediately after curing, the UV laminated sheets can be stacked or stored on rolls without a loss in quality. This is a precondition for the continuous operating mode. Depending on the projected purpose, very different requirements are imposed on lamination, which in itself represents an enhancement of the base material.
In a first category of lamination, its principal purpose is to increase mechanical strength and protect the surface of the imprinted base material, which in this case may be imprinted paper or cardboard. Book jackets, picture portfolios, posters, etc. may be produced from the latter.
In a second category, lamination is required particularly for improving or enhancing the effect of colors or a print. The end product is a coated material which has been enhanced by a transparent film, e.g. for brochures or other advertisements and high-price art prints. This produces a glass-like effect. Any disturbing secondary effect such as very small air bubbles between the film and the base material, inadequately joined locations or striated effects in the applied coat must be avoided particularly where requirements are exacting, as in art prints and superior advertising printing. The content of the print should be enhanced, but never disrupted by secondary results of lamination. On the contrary, any defects such as surface roughness should be concealed or covered prior to lamination. Accordingly, the visual impression is also subject to the most exacting requirements. In this regard, it is well known that the perceptive ability of the human eye is almost unsurpassed, so that these requirements border on near perfection.
It has recently been shown that a solution according to DE-PS 2 817 917 enables an optimal "protective lamination" with great success. On the other hand, this solution cannot be used commercially for "art lamination" in view of the high expectations associated with the latter, since very small disturbing effects simply cannot be avoided with sufficient operating dependability. Even very large-scale, lengthy investigations have not achieved the desired success, particularly also in the selection of specific adhesives, film materials, etc. From the vantage point of the characteristics of radiation-curable adhesive, the adhesive fastening is now no longer the chief factor hindering processing speed.